FIGURE 14.

Working hypothesis of uridine nucleotide signaling in human EC cells lining the GI mucosa. UTP (or UDP) activate P2Y₄ and P2Y₆ receptors in EC cells to elevate intracellular free Ca²⁺ levels ([Ca²⁺]_i) and generate Ca²⁺ oscillations or monophasic Ca²⁺ transients, and cause membrane depolarization. UTP acts via P2Y₄ to stimulate a Gq/PLC-PIP2-IP3-IP3R/SERCA signaling pathway to elevate [Ca²⁺]_i associated with Ca²⁺ responses that triggers a Ca²⁺ dependent 5-HT secretion in EC cells. In addition, patch-clamp studies revealed that UTP increases I_Ca currents by activation of VOCCs via P2Y₄; P2Y₆ is presumed to have an inhibitory effect on VOCCs. As well, activation of P2Y₄ causes a V_m depolarization that was independent of VOCC activation, but was eliminated by the K_v7.1/7.2/7.3 channel inhibitor XE-991. We hypothesize that UTP depolarizes the cell by reducing a potassium conductance (not yet identified), which occurs as a consequence of depleting PIP2 from the membrane by its conversion to IP3 after PLC activation. V_m depolarization seems not linked to Ca²⁺ dependent 5-HT release but it may play a role in the fine tuning of the pathway. In a parallel way UTP reduced a component of I_K currents that was insensitive to XE-991, and the identity of the K_v channel remains unknown. This response does not explain the effect of UTP on V_m depolarization. A rise in intracellular free Ca²⁺ is required for 5-HT release. However, zero Ca²⁺ buffer augments both [Ca²⁺]_i inside the cell and 5-HT release. The mechanism for this paradoxical effect of Ca²⁺ influx is unknown. Concepts are based on data from a BON cell model of human EC cells. Isolated EC cells from human intestinal surgical specimens were used to confirm effect of UTP on 5-HT release and expression of P2Y₄ and P2Y₆ receptors.